The Strange Science Of Sexual Attraction

While we'd like to think that we know why a particular person catches our eye, there are a number of invisible forces at work that determine which members of the opposite sex we become interested in -- and which ones we don't.

Of course, there are a number of factors that go into who we choose to be with, including personality traits, interests and values and physical appearance. But when it comes to immediate, knee-jerk physical attraction, we often can't pinpoint why exactly we're drawn to someone. Even as scientific research has shed more light on the factors that contribute to our selection of a sexual mate, the biology of attraction is complex and not yet fully understood -- and it doesn't help that attraction is particularly difficult to replicate in a lab.

So what really is happening when the sight of a hot guy or girl makes us instantly swoon? Human biology and evolutionary psychology has some answers.

Here are some of the subtle but powerful factors that may help determine who we're attracted to.

We fall in love at first "smell."

"Smell" is the woefully inadequate way we describe sensing someone's pheromones -- a type of scent-bearing chemical secreted in sweat and other bodily fluids. Pheromones are known to be involved in sexual attraction in animals, and research suggests that they may also play a role for people. A type of pheromone called a "releaser" -- which includes the compounds androstenone, androstadienone and androstenol -- may be involved in sexual attraction, according to a Reactions: Everyday Chemistry video.

In one study, female participants were tasked with the unpleasant directive to smell men's sweaty undershirts. The researchers found that women could smell how symmetrical a man was, and using that information, judged his attractiveness. (In both men and women, symmetry is known to be an important factor in attractiveness.)

Men can detect a fertile woman.

Men can actually sense fertility on a woman, perhaps due in part to her pheromones. During the most fertile time in her menstrual cycle, a woman gives off a different scent which may make her more attractive to potential male suitors. Research from the University of Texas at Austin investigated this phenomenon by asking a group of women to wear T-shirts to sleep during both fertile and infertile points in their cycles, and then asked men to smell the T-shirts and assess which ones they found most pleasing. Overwhelmingly, they judged the shirts worn by the fertile women to be more "pleasant" and "sexy."

A woman's face may also appear more attractive to men during the most fertile point in her cycle. A British study conducted in 2004 asked a group of 125 men to look at two pictures of the same woman, at times of high and low fertility in her cycle, and to assess which photo was more attractive. Nearly 60 percent of the men rated the photos of the women's faces at peak fertility (eight to 14 days after her last period) to be more attractive.

The sound of a woman's voice also plays into a man's judgements of a woman's attractiveness. A recent study found that a woman's voice sounds most seductive at the most fertile point in her menstrual cycle -- and that hearing a woman's peak-fertility voice can literally make a man's skin tingle.

Women quickly assess markers of masculinity.

A large body of evolutionary psychology research has shown that, in general, women tend to prefer more masculine-looking men -- perhaps because masculine features like broad shoulders or a strong jawline are indicators of virility and good health. But today, this doesn't always hold true.

Women may have evolved to seek out virility, but that doesn't mean that their preference in a modern context is always for "manly" men (and ditto for men's attraction to "fertile looking" women). Not all -- or even the majority -- of women prefer more masculine men. One study found that context matters: Women living in poorer environments may have a greater preference for masculine men, but women in more developed areas prefer more feminine-looking men, according to a study from the Face Research Laboratory.

"From an evolutionary perspective, masculinity is basically man's way of advertising good genes, dominance and likelihood to father healthier kids," the Wall Street Journal explained. "When disease is a real threat, as it had been—and arguably still is—heritable health is invaluable."

One time this preference may hold true is when a woman is at the most fertile point in her cycle. One study found that women whose partners had less masculine facial features reported attraction to more masculine-looking men when they were ovulating. However, women whose partners had more masculine features did not report the same eye-wandering. However, these findings only applied to women in short-term relationships -- not serious, committed partnerships.

The Pill might change a woman's preference in men.

Is she really attracted to you -- or is it just her birth control? A number of studies have suggested that hormonal contraception may have some effect on women's preferences for sexual partners.

A man's smell provides a woman with information about his major histocompatibility complex (MHC) genes, which play an important role in immune system function. As the thinking goes, women prefer men whose MHC genes differ from their own because children with more varied MHC profiles are more likely to have healthy immune systems -- which makes a whole lot of sense from an evolutionary perspective. However, research has shown that women on the pill actually display a preference for men with more similar MHC genes to their own. Scientists aren't entirely sure why this happens, but one hypothesis is that the hormonal changes involved in pregnancy (which the Pill mimics) might draw women more to "nurturing relatives."

Even within long-term committed relationships, changes in hormonal contraception use might affect a woman's sexual satisfaction with her male partner. "Women who had met their partner while taking the pill and were still currently taking it -- as well as those who had never used the pill at any point -- reported greater sexual satisfaction than those women who had begun or stopped using the pill during the course of the relationship," lead researcher Dr. Craig Roberts said in a statement.

But personality is important, too.

It's not all about a person's looks and their chemical makeup -- certain personal characteristics can also play a role in determining how "hot or not" someone is.

Kindness, for instance, can make a person more attractive in addition to making them more likable. A 2014 study found that positive personality traits actually increase perceived facial attractiveness. The researchers asked 120 participants to rate 60 photographs of female faces in neutral expressions. Two weeks later, they were asked to evaluate the same photos, but this time, half of the photos were accompanied by positive personality descriptors like kind and honest, and half of which were accompanied by negative descriptions like mean and dishonest. A control group saw the photos without any descriptions.

The photographs with the positive personality descriptions received the highest ratings for facial attractiveness, while the group with negative descriptions was ranked as less attractive than both the negative and the control group.

"We find that 'what is good is beautiful,' with personality reflecting desired traits as facial attractiveness," the researchers wrote. "This phenomenon can also be called the 'halo effect.' We can thus presume that personality traits may contribute to judging facial attractiveness and that the personality traits desired in a person are reflected in facial preference."

Who we're attracted to is still a very individual matter.

While there is something of a science to the romantic and sexual partners we choose, at the end of the day, attraction is still completely unique to each of our individual makeups and preferences.

Anthropologist Helen Fisher, who has studied love and dating extensively, explains that we each have individual "love maps" that determine who we gravitate towards.

"These love maps vary from one individual to the next. Some people get turned on by a business suit or a doctor's uniform, by big breasts, small feet, or a vivacious laugh," Fisher writes in Psychology Today, adding, "But averageness still wins."

Fisher cites a study in which participants selected faces of 32 women, and used a computer program to make their features look more average. Then, they showed these photos as well as 94 photos of real female faces to a group of college students. Only four of the photographs of real female faces were rated as more attractive than the "averaged" faces.

As Fisher suggests, while individuals and cultures have their own standards for what they consider attractive, there are some fairly universal qualities that we all look for, including a clear complexion, symmetrical faces, wide hips (for women), and a general appearance of health and cleanliness.